There’s quite a knack to opening oysters, but once you have it you have it! Just make sure, if you don’t have an actual oyster knife, that your knife tip is pointed but not sharp and is a good thickness so it won’t snap. Our knife (see the photo) is really good.
Put a tea towel in the palm of your hand and the oyster on top, flat side facing upwards. Make sure the tea towel is covering your hand as it’s very easy to slip with the knife.
Put the tip of the oyster knife into the narrow hinge end of the oyster, along the line where the two shells meet. Gently but firmly push the knife in and twist, opening up the shell, then slide your knife along, keeping it tight against the inside of the upper shell to cut the oyster where it connects to the shell.